By Paul Rogers
Hybridizer. See examples of my hybids at about 7 months, 18 months,and 3 years. Serious home gardeners can see the mini-greenhouses I use, basically made from bent 10' sections of electrical conduit. This picture shows them in the summer configuration, covered with 50% shade cloth. It also shows a mini-lathhouse from 3 sheets of commercial cedar lattice. This picture shows them with clear plastic ends attached, ready for colder weather. The milk jugs are filled with water as my attempt to increase "thermal mass". This picture shows them on a clear and frosty but calm morning with the transparent plastic cover unrolled over the top, but the "shirt-tails" sticking out. During a snowstorm I can tuck in the "shirt-tails", and button them up snug. During summer I have light frames covered with 50% shade cloth for plants in full sun. The legs are adjustable for height, and when the weather changes they are easily disassembled.
Rhododendrons are wonderful plants. They are glorious bloomers. Relatively disease free. And relatively low maintenance. But there are a few things gardeners should know.
Things every gardener should know about Rhododendrons.
Above all, rhododendrons are very shallow rooted. The roots form a dense fibrous mat under the dripline. This one fact affects most aspects of gardening with rhododendrons.
What's the difference between a rhododendron and an azalea?
In short, not much. Both are members of the Genus Rhododendron. The taxonomists have a set of rules, that sometimes change, for dividing them. You can just consider there are a couple different species that are called rhododendrons, and a couple that are called azaleas.
If you want a simple, generally reliable, way to tell when you're looking at one, azaleas have one stamen per petal lobe, always five lobes. Rhododendrons have two stamens per petal lobe, and some have more than five lobes.
Immediately after blooming rhododendrons should be dead-headed, the spent flower trusses should be picked or cut off. This one thing will do the most to keep your rhododendrons growing and blooming at their best.
Here is a good example of the "truss" structure on which most rhododendrons bloom. Note the brown, scaley section where truss attaches to the branch. This is generally a weak spot there the spent truss can be snapped off. I prefer to use a small, needle-nosed pruner to make a clean cut.
It's a good idea to wear gloves—they produce a sticky substance that is difficult to remove. Neither soap and water, nor many hand cleaners, seems to work very well. (WD-40 dissolves it well enough, but I prefer not to wash my hands with it.) Helen tells me mineral oil works with a little effort.
Immediately after blooming is when rhododendrons should be pruned.
Otherwise you may cut off next year's bloom. But some varieties bud
freely and others hardly at all. Have a care until you know which yours is.
Usually about the time of blooming and pruning, when the new growth is in the "candle" stage you should pinch off many of the single candles to promote branching, if your rhododendron buds readily enough. Sometimes two candles develop more or less equally. That's a good thing. There's a balance to be struck here: some rhodys won't flower as readily on the resulting branches, some do, some develop two or more buds readily on their own, some don't. So pinch enough to keep the plant full and well branched, but no more than necessary.
Here is an example of "Avalanche" showing a flower bud soon to pop, and a young growth bud not quite to candle stage yet. If left it would have elongated, burst out with leaves, then the branch would have extended. The vestigial buds would have remained undeveloped. Snapped off, the vestigial buds will begin to grow, developing two or more new branches. (We hope.)
If you don't get to them soon enough, and the leaves begin to come out, I wouldn't pinch them. The vestigial buds may not develop fully then. Below you can see the result of pinching last year.
Several weeks later we see there are two new buds growing rapidly.
This time they stemmed from vestigial buds right under the original. The
remaining vestigial buds in the lower leaves are still undeveloped. They are
still useful. Suppose we hadn't pinched out the original bud, and two or
three years in the future of single buds extending we decide to restructure
the branch. This joint, or close to it, would be a candidate pruning point.
That vestigial bud at the base of the leaf will be a good candidate for
development. The older they get the less likely, but it's there and it's nice
and plump. On some you wouldn't even see a vestigial bud like this. I might
take the chance.
Rhododendrons are in the nurseries and home centers in the spring, when they are in bloom. This is the best time to make your selections. But this is not the best time to buy and plant new rhododendrons! Remember what I said about being subject to stress during the dry summer? That's tough on new transplants. Here in Zone 8 at least, the best time to buy and plant the selections you made in the spring is in the fall when the rains have returned. This helps relieve transplant stress and gives them a chance to begin to establish a root system before winter, and certainly before next summer. Most local nurseries won't have them then—people want to buy blooming plants and Rhododendrons bloom in spring. So you might have to get them from a grower, and might even get better plants that way. If you have to plant them in spring, make sure you water them especially well that first summer or two until they are well established.
The exceptions to that advice are the deciduous azaleas. Being dormant all winter makes them too susceptible to root rot. Plant them in early spring. Preferably just before they "bud-out".
Some hybrids are susceptible to root weevils, most relatively resistant. The leaf edges are notched, like someone with a hole-punch. Spraying with a systemic like Orthene works, because the critters are nocturnal.
The biggest mistake I think people make is to ignore the mature size of the variety they have selected in the gallon pot. Some varieties of Rhododendrons may never get knee-high, others will get the size of trees. Get a book or grower's catalog and check it out. I recommend Greer's Guidebook to Available Rhododendrons.
About specific choices, first know that there is no true blue in rhodys. To call Blue Jay or the like anything more than somewhat lavender is over-generous in my opinion. Likewise, the yellows lack brillance in the evergreens, look to the deciduous Exbury azaleas. There are good whites, but many whites have faint blushes of pink or yellow, especially in bud. Rhodys excel in pinks of all shades; light pinks, dark pinks, purply pinks, intense pinks. But we also have good, true reds—so don't settle for an intense pink if what you want is a red.
Copyright © 1999, 2000, 2003 by Paul Rogers. All rights reserved.